Effect of Humidity on Posterior Lens Opacification During Fluid-Air Exchange

Objective  To study the relationship of humidity and the rate of lens opacity formation during fluid-air exchange using an animal model.

Methods  Vitrectomy and fluid-air exchange was carried out using 16 eyes of 8 pigmented rabbits. One eye of each rabbit was exposed to dry air and the fellow eye received humidified air using an intraocular air humidifier. In each case, the percent humidity of the intraocular air was measured using an in-line hygrometer. Elapsed time from initial air entry to lens feathering was recorded for each eye, with the surgeon-observer unaware of the percent humidity of the air infusion.

Results  In each rabbit, use of humidified air resulted in a delay in lens feathering (P<.02), with an overall increase in time to feathering of 80% for humidified air vs room air.

Conclusions  Use of a humidifier during fluid-air exchange prolongs intraoperative lens clarity in the rabbit model, suggesting that humidified air should prolong lens clarity during phakic fluid-air exchange in patients.

Clinical Relevance  Use of humidified air during vitrectomy and fluid-air exchange may retard the intra-operative loss of lens clarity, promoting better visualization of the posterior segment and enhancing surgical performance.

LOSS OF LENS CLARITY during vitrectomy and fluid-air exchange is a well-known phenomenon to experienced vitreoretinal surgeons, although few formal descriptions exist in the literature. The loss begins as a subtle, feathery posterior subcapsular opacification, often with a granular or placoid vacuolar character. As this occurs, fine fundus details may be lost, compromising diagnostic evaluation and surgical performance.

Although to our knowledge no formal studies have specifically addressed such intraoperative feathering of the posterior lens, permanent postoperative posterior subcapsular changes have been shown to correlate with the duration of vitrectomy surgery and the presence and volume of intravitreal gas.^1-3 Some authors^4,5 postulate that direct contact of intraocular gas with the posterior lens surface somehow interferes with lens metabolism and nutrition, although the exact pathophysiological mechanism remains unknown.

We speculate that the posterior lens opacification observed during intraocular air delivery is caused by desiccation of the posterior lens surface as it comes in contact with the air bubble. Drying out of the posterior lens capsule and underlying epithelium may initiate some form of metabolic or osmotic disturbance of the epithelial layer, leading to subcapsular feathering. The theory of dehydration as a possible cause of cataract formation dates back to Goldmann and Rabinowitz,⁶ who postulated that acute postmortem cataracts were the result of a process of transcorneal water evaporation that left the aqueous humor dehydrated and hyperosmolar. Fraunfelder and Burns⁷ later demonstrated the effectiveness of increased external humidity in the prevention or delay of anterior subcapsular lens opacities in animal models. The present study of humidified fluid-air exchange in a rabbit model was undertaken to further explore this hypothesis in the context of the internal ocular microenvironment.

All pigmented rabbits (Gingrich Animal Supply, Fredericksburg, Pa) used in the study were treated according to the Association for Research in Vision and Ophthalmology resolution on the use of animals in research. Eight rabbits each underwent 2-port vitrectomy and fluid-gas exchange after intramuscular injection of ketamine hydrochloride (12 mg/kg) and xylazine hydrochloride (6 mg/kg) (Phoenix Pharmaceutical Inc, St Joseph, Mo). A thorough core and peripheral vitrectomy was carried out in each eye, taking care to remove as much retrolenticular vitreous as possible without damaging the lens. Total operative time for the core vitrectomy was 4 minutes, followed by an additional 4 minutes for the peripheral vitrectomy. This was followed by a standard fluid-air exchange using dry air in a randomly assigned eye and humidified air in the fellow eye. A microsurgical system (Storz, St Louis, Mo) referred to as machine 1, was used in half of the rabbits to supply dry tank air (humidity <5%). A second surgical system (Storz), referred to as machine 2, was used for the remaining half to supply dry room air (humidity 20%-30%). Humidified air was generated using an intraocular air humidifier (American Medical Devices Inc, Atlanta, Ga) and placed in-line along the air tubing (Figure 1). The air humidifier was prepared before fluid-air exchange by injecting 2 to 3 mL of clean, nonsterile water into the filter within the air humidifier assembly. This allowed all surfaces of the internal filter to be completely moistened in order to humidify dry air passing through the filter; any excess fluid was drained from the humidifier. Intraocular humidity was measured using a digital hygrometer (Fisher, Pittsburgh, Pa) placed along the air supply. The single surgeon-observer was unaware of the percent humidity value of the air infusion in all cases, and there were no obvious visual or auditory cues that allowed the surgeon to distinguish between humidified air delivery and dry air delivery. Using a handheld contact lens (Ocular Instruments Inc, Bellevue, Wash), feathering was judged by focusing the operating microscope (Storz) on the posterior lens surface under high magnification (Figure 2). Forceps were used to position the globe so that glare and troublesome light reflexes could be minimized. Length of time from gas entry to the moment at which feathering was detected by the surgeon-observer was recorded for each eye. Each animal was killed with a 3-mL intracardiac injection of pentobarbital sodium and phenytoin sodium (Schering-Plough Animal Health Corp, Kenilworth, NJ) at the conclusion of the operation.

Table 1 summarizes the data obtained during the study. Dry air varied from 19% to 30% humidity with dry room air supplied by machine 2, in contrast to the consistent 3% humidity value found with dry tank air supplied by machine 1. Figure 3 is a representative graph showing a comparison of measured humidity for dry tank air, room air, and humidified tank air. For humidified tank air, after an initial peak, the humidity maintained a relatively constant level for 20 minutes. Although not shown in Figure 3, humidified room air produced even higher levels of relative humidity compared with humidified tank air. In each of the 8 rabbits, use of humidified air during fluid-air exchange correlated with an increase in time to lens feathering (P<.02), but there was no linear relationship between level of humidity and time to lens feathering. In Figure 4, time to feathering with humidified air is plotted against time to feathering with dry air (both room and tank air) for each of the 8 pairs of eyes. The line of regression demonstrates a linear relationship (slope=1.80, r²=0.83) that shows an 80% increase in the time to feathering for eyes undergoing humidified fluid-air exchange.

The data generated in this study suggest that humidified air retards the development of posterior lens opacities during fluid-air exchange, extending operating time and maintaining crisp visualization of fundus details. Although the study lends indirect support to the hypothesis that posterior lens desiccation may be the initiating factor for posterior lens feathering, further investigations are needed to characterize the pathogenesis of posterior lens changes in the setting of vitrectomy surgery.

Some potential pitfalls of this preliminary study include falsely delayed feathering from inadequate retrolenticular vitrectomy and falsely accelerated feathering becasue of inadvertent lens trauma. In addition, detection of the first signs of posterior feathering by a single unbiased surgeon-observer still involves a subjective judgment in the absence of any definite quantifiable characteristics. Despite these potential confounders, this preliminary series provides a basis for further study into the use of humidification and other modifications of the intraocular microenvironment during vitreoretinal surgery.

Accepted for publication March 1, 1999.

The intraocular air humidifier used is available from American Medical Devices Inc, Atlanta, Ga.

Reprints: Eugene de Juan, Jr, MD, Johns Hopkins University Microsurgery Advanced Design Laboratory, Wilmer Eye Institute, Johns Hopkins University Hospital, 721 Maumenee Bldg, 600 N Wolfe St, Baltimore, MD 21287 (e-mail: edejuan@jhmi.edu).